My sis got married recently and the night they were engaged, my mom and I were excluded. why?

Since you can pinpoint the beginning of your sister's aberrant behavior to the coincidence of her engagement to her now-husband, it's reasonable to conject that her ociation with his family is somehow the cause. Sadly this type of thing is not particularly uncommon, but if it helps you to know this, people usually revert back to their normal selves after life kicks them in the a s s a little bit. Trying to figure out exactly why won't do much for you except frustrate you, because ultimately there is no good reason to turn your back on your family. But we've all been guilty of it to some degree at one time or another, and it invariably pes leaving us humbled enough to see who's really got our backs. So, let it ride for now (and maybe just accept it as an annoying rite of page you sister just has to go through to get over). When life has dealt with her sufficiently, she'll be back, and you'll get a better sister in the process. As best as you and your mom can manage, continue to be there for her...good luck in this endeavor.